1. Field of the Invention
One disclosed aspect of the embodiments relates to a luminescent device capable of displaying at least two different colors, and particularly to an organic electroluminescent (EL) device including at least a first EL element and a second organic EL element, each of which emits light of a different color.
2. Description of the Related Art
In a method for manufacturing an organic EL device capable of displaying a plurality of colors, a shadow mask is widely used to selectively form a luminescent layer that emits a specific color in a specific area. Meanwhile, high-definition luminescent devices exceeding 300 pixels per inch (ppi) has been recently demanded, and it is becoming increasingly difficult to manufacture such luminescent devices by using the conventional shadow mask. Japanese Patent No. 4578026 discusses, as a technique capable of miniaturization, a method in which luminescent layers are selectively formed by patterning using photolithography.
Japanese Patent No. 4578026 discusses a method in which a desired pattern of resist is formed on a luminescent layer by photolithography, and the luminescent layer in the areas where the resist is not formed is removed by wet etching or dry etching. In this way, luminescent layers may be selectively formed. An end portion of the luminescent layers patterned in such method may be formed to have a smaller inclined area, compared with an end portion of the luminescent layers selectively formed by using a shadow mask. The inclined area is an area where the thickness of the film is decreased toward an end portion of the film. In other words, the inclined area is an area that is inclined with respect to a substrate.
If a film is formed by using a shadow mask, depending on the distance between the shadow mask and the film formation surface and on the incident angle of deposited particles incident on the film formation surface, an inclined area having a width several times to several dozen times the thickness of the formed film is formed at a film end portion that is formed at an opening of the shadow mask. In contrast, if a film is patterned by using photolithography and etching, the inclined area may be formed to have a width less than or equal to the thickness of the formed film.
In particular, if dry etching is used, since the etching may be performed in the approximately vertical direction with respect to the substrate, a film end portion may have a large inclination angle of 60 to 90 degrees with respect to the substrate, and the inclined area may be formed to have a width narrower than the film thickness. If luminescent layers arranged in an element area of the luminescent device are patterned by photolithography, the distance between luminescent layers adjacent to each other may be reduced. Thus, organic EL elements may be densely arranged. This is advantageous in reducing the size of the luminescent device and improving the aperture ratio of the light emitting elements. In one embodiment, the element area means an area where a plurality of display-related organic EL elements is formed. In addition, the thickness of the formed film means an average of the film thickness measured at a plurality of positions other than the inclined area.
Power for driving the luminescent device is supplied from an external power source via an external connection terminal previously provided outside the element area. Two electrodes for the external connection terminal and the organic EL elements are electrically connected via a wiring. A switching element such as a transistor may be provided in the wiring, as long as the two electrodes for the external connection terminal and the organic EL elements may be electrically connected when necessary. Generally, the wiring and a second electrode formed after the luminescent layers are patterned are electrically connected via a contact portion. The contact portion is formed outside the element area by removing high-resistance material such as that of a luminescent layer formed on a wiring layer. In addition, organic material such as that of a luminescent layer is removed from the surface of the external connection terminal so that the external connection terminal is electrically connected to the external power source. The organic material is also removed such that the removed area surrounds the luminescent area so that a pathway of moisture intruding from the outside to the organic EL elements is cut off.
If the luminescent layer outside the element area (such as the contact portion) is removed simultaneously when the luminescent layers within the element area are patterned, the manufacturing process may be advantageously simplified. However, the end portions of the luminescent layers outside and inside the element area are etched under the same conditions, the end portions are formed similarly to the inside the element area to have a large inclination angle with respect to the substrate. If an area having a large inclination angle with respect to the substrate exists on the surface on which the second electrode is to be formed, the second electrode could be disconnected or the film thickness of the second electrode could be thinned, then resistance may increase. Particularly, when the second electrode is formed by a high-directivity deposition method or is formed to have a thin film thickness, if an area having an inclination angle as large as 90 degrees exists on the surface on which the second electrode is formed, the second electrode is highly likely to have high resistance. If the second electrode is disconnected or the film thickness is thinned, a current flowing from the second electrode to the wiring layer through a luminescent element is subjected to high wiring resistance. Consequently, various problems are caused. In some cases, the luminescent device does not turn on. In other cases, the luminescent device exhibits defective light emission such as shading or undergoes a voltage increase.